1. Field of the Invention
The present invention relates to a dynamic bearing device for supporting a rotary member in a non-contact manner by a dynamic pressure action of a fluid (i.e., lubricating fluid) generated in a bearing gap. The dynamic bearing device is suitable for use in: a spindle motor for an information apparatus, for example, a magnetic disk device, such as an HDD or an FDD, an optical disk device, such as a CD-ROM, a CD-R/RW, or a DVD-ROM/RAM, or a magneto-optical disk device, such as an MD or MO; a polygon scanner motor for a laser beam printer (LBP); or a small motor for an electrical apparatus such as an axial fan.
2. Description of the Related Art
Apart from high rotational accuracy, an improvement in speed, a reduction in cost, a reduction in noise, etc. are required of the various motors mentioned above. One of the factors determining such the requisite performances is a bearing for supporting a spindle of the motor. Recently, as a bearing of this type, use of a dynamic bearing superior in the above-mentioned requisite performances has been considered, or such the dynamic bearing has been actually put into practical use.
For example, in a dynamic bearing device to be incorporated into the spindle motor of a disk drive apparatus, such as an HDD, there are provided a radial bearing portion supporting a shaft member radially in a non-contact manner and a thrust bearing portion supporting the shaft member in a thrust direction in a non-contact manner. As the radial bearing portion, there is used a dynamic bearing provided with grooves (i.e., dynamic pressure grooves) for dynamic pressure generation provided in an inner peripheral surface of a bearing sleeve or in an outer peripheral surface of the shaft member. As the thrust bearing portion, there is used a dynamic bearing provided with dynamic pressure grooves in, for example, both end surfaces of a flange portion of the shaft member or in surfaces opposed thereto (e.g., an end surface of the bearing sleeve and an end surface of a thrust member fixed to a housing (see, for example, Patent Documents 1 and 2). Alternatively, as the thrust bearing portion, there may be used a bearing (i.e., a so-called pivot bearing) of a structure in which one end surface of the shaft member is contact-supported by a thrust plate (see, for example, FIG. 4 of Patent Document 2).
In general, the bearing sleeve is fixed to a predetermined position of an inner periphery of the housing and, to prevent leakage of the fluid (e.g., a lubricating oil) poured into an inner space of the housing to the outside, a seal member is arranged at an opening of the housing in many cases. The inner peripheral surface of the seal member defines a seal space between itself and the outer peripheral surface of the shaft member, and the volume of the seal space is set to be larger than the amount by which the lubricating oil filling the inner space of the housing undergoes a change in volume through thermal expansion/contraction within a temperature range of use. Thus, even when there is a change in the volume of the lubricating oil as a result of a temperature change, an oil level of the lubricant is always maintained within the seal space (see Patent Document 1).    Patent Document 1: JP 2003-65324 A    Patent Document 2: JP 2003-336636 A
As described above, in the conventional dynamic bearing device, the seal space is formed between the inner peripheral surface of the seal member fixed at the opening of the housing and the outer peripheral surface of the shaft member; if the seal space is to have a function to absorb a change in the volume of the lubricating oil due to a temperature change, it is necessary to secure a relatively large axial dimension for the seal space (i.e., the seal member). Thus, from the design standpoint, it is necessary to lower, within the housing, the position of the axial center of the bearing sleeve relatively toward a bottom side of the housing, with the result that the distance between the bearing center of the radial bearing portion and the center of gravity of the rotary member increases, which, depending upon the condition of use, etc., can lead to a shortage of load capacity with respect to a moment load. Further, in a construction in which thrust bearing portions are provided on both sides of the flange portion of the shaft member, the axial distance between the two thrust bearing portions becomes relatively smaller, with the result that the load capacity of the thrust bearing portions with respect to the moment load tends to be so much the lower. In particular, in a case of a dynamic bearing device for use in a disk drive apparatus, as a rotor (i.e., a rotary member to which a rotor hub, a rotor magnet, a disk, a clamper, etc. are assembled) rotates, a relatively large moment load acts on the shaft member, so the moment load resistance is an important characteristic.
Further, in a dynamic bearing of this type, the thrust bearing gap of the thrust bearing portion is under the influence of component precision, assembly precision, etc., so it is difficult to control the thrust bearing gap to a desired value. Under the circumstances, there is nothing for it but to perform a complicated assembly operation.
It is an object of the present invention to make it possible to reduce the axial dimension of the above-mentioned seal space of a dynamic bearing device of this type, thereby enhancing the load capacity of the dynamic bearing device with respect to the moment load or reducing the axial dimension of the dynamic bearing device.
Another object of the present invention is to enhance the load capacity of the thrust bearing portion with respect to the moment load.
Still another object of the present invention is to provide a method which makes it possible to easily set the thrust bearing gaps of a dynamic bearing device of this type with high accuracy.